Make globals used for array initialization codegen constant

[hans]

Summary

As pointed out in D133835 these globals will never be written to
(they're only used for trivially copyable types), so they can always be
constant.

[efriedma]

LGTM

clang/lib/CodeGen/CGExprAgg.cpp

if (NumInitElements * elementSize.getQuantity() > 16 &&
       elementType.isTriviallyCopyableType(CGF.getContext())) {
     CodeGen::CodeGenModule &CGM = CGF.CGM;
     ConstantEmitter Emitter(CGF);
     LangAS AS = ArrayQTy.getAddressSpace();
     if (llvm::Constant *C =
             Emitter.tryEmitForInitializer(ExprToVisit, AS, ArrayQTy)) {
       auto GV = new llvm::GlobalVariable(
           CGM.getModule(), C->getType(),
-          CGM.isTypeConstant(ArrayQTy, /* ExcludeCtor= */ true,
-                             /* ExcludeDtor= */ false),
-          llvm::GlobalValue::PrivateLinkage, C, "constinit",
+          /* isConstant= */ true, llvm::GlobalValue::PrivateLinkage, C,
+          "constinit",
           /* InsertBefore= */ nullptr, llvm::GlobalVariable::NotThreadLocal,
           CGM.getContext().getTargetAddressSpace(AS));
       Emitter.finalize(GV);
       CharUnits Align = CGM.getContext().getTypeAlignInChars(ArrayQTy);
       GV->setAlignment(Align.getAsAlign());
       Address GVAddr(GV, GV->getValueType(), Align);
       EmitFinalDestCopy(ArrayQTy, CGF.MakeAddrLValue(GVAddr, ArrayQTy));
       return;

clang/test/CodeGen/init.c

 // RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm %s -o - | FileCheck %s
 
 struct I { int k[3]; };
 struct M { struct I o[2]; };
 struct M v1[1] = { [0].o[0 ... 1].k[0 ... 1] = 4, 5 };
 unsigned v2[2][3] = {[0 ... 1][0 ... 1] = 2222, 3333};
 
 // CHECK-DAG: %struct.M = type { [2 x %struct.I] }
 // CHECK-DAG: %struct.I = type { [3 x i32] }
 
 // CHECK-DAG: [1 x %struct.M] [%struct.M { [2 x %struct.I] [%struct.I { [3 x i32] [i32 4, i32 4, i32 0] }, %struct.I { [3 x i32] [i32 4, i32 4, i32 5] }] }],
 // CHECK-DAG: [2 x [3 x i32]] {{[[][[]}}3 x i32] [i32 2222, i32 2222, i32 0], [3 x i32] [i32 2222, i32 2222, i32 3333]],
-// CHECK-DAG: [[INIT14:.*]] = private global [16 x i32] [i32 0, i32 0, i32 0, i32 0, i32 0, i32 17, i32 17, i32 17, i32 17, i32 17, i32 17, i32 17, i32 0, i32 0, i32 0, i32 0], align 4
+// CHECK-DAG: [[INIT14:.*]] = private constant [16 x i32] [i32 0, i32 0, i32 0, i32 0, i32 0, i32 17, i32 17, i32 17, i32 17, i32 17, i32 17, i32 17, i32 0, i32 0, i32 0, i32 0], align 4
 
 void f1(void) {
   // Scalars in braces.
   int a = { 1 };
 }
 
 void f2(void) {
   int a[2][2] = { { 1, 2 }, { 3, 4 } };

clang/test/CodeGen/label-array-aggregate-init.c

 // RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm %s -o - | FileCheck %s
 
-// CHECK: @constinit = private global [3 x ptr] [ptr blockaddress(@main, %L), ptr null, ptr null]
+// CHECK: @constinit = private constant [3 x ptr] [ptr blockaddress(@main, %L), ptr null, ptr null]
 
 void receivePtrs(void **);
 
 int main(void) {
 L:
   receivePtrs((void *[]){ &&L, 0, 0 });
 }